fix doc deploy (#132)

That cachix version doesn't exist anymore

Author: zimbatm

.github/workflows/gh-pages.yml

@@ -21,11 +21,7 @@ jobs:
             - uses: actions/checkout@v5
               with:
                   fetch-depth: 0
-            - uses: cachix/install-nix-action@V27
-              with:
-                  extra_nix_config: |
-                      accept-flake-config = true
-                      experimental-features = nix-command flakes
+            - uses: cachix/install-nix-action@v31
             - name: Configure Git user
               run: |
                   git config --local user.email "github-actions[bot]@users.noreply.github.com"